Extreme ultraviolet lithography process and mask

ABSTRACT

An extreme ultraviolet lithography (EUVL) system is disclosed. The system includes an extreme ultraviolet (EUV) mask with three states having respective reflection coefficient is r 1 , r 2  and r 3 , wherein r 3  is a pre-specified value that is a function of r 1  and r 2 . The system also includes a nearly on-axis illumination (ONI) with partial coherence σ less than 0.3 to expose the EUV mask to produce diffracted light and non-diffracted light. The system further includes a projection optics box (PUB) to remove a portion of the non-diffracted light and to collect and direct the diffracted light and the remaining non-diffracted light to expose a target.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 14/047,341, entitled “An Extreme Ultraviolet LithographyProcess and Mask,” filed Oct. 7, 2013, the entire disclosure of which ishereby incorporated by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. In the course of IC evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs. Suchscaling down has also increased the complexity of IC processing andmanufacturing. For these advances to be realized, similar developmentsin IC processing and manufacturing are needed. For example, the need toperform higher resolution lithography processes grows. One lithographytechnique is extreme ultraviolet lithography (EUVL). Other techniquesinclude X-Ray lithography, ion beam projection lithography, electronbeam projection lithography, and multiple electron beam masklesslithography.

EUVL employs scanners using light in the extreme ultraviolet (EUV)region, having a wavelength of about 1-100 nm. Some EUV scanners provide4× reduction projection printing, similar to some optical scanners,except that the EUV scanners use reflective rather than refractiveoptics, i.e., mirrors instead of lenses. EUV scanners provide thedesired pattern on an absorption layer (“EUV” mask absorber) formed on areflective mask. Currently, binary intensity masks (BIM) accompanied byon-axis illumination (ONI) are employed in EUVL. In order to achieveadequate aerial image contrast for future nodes, e.g., nodes with theminimum pitch of 32 nm and 22 nm, etc., several techniques, e.g., theattenuated phase-shifting mask (AttPSM) and the alternatingphase-shifting mask (AltPSM), have been developed to obtain resolutionenhancement for EUVL. But each technique has limitations. For example,for AltPSM, one of the methods to generate a phase-shifting regionwithout significant attenuation in reflectivity is to create a step ofappropriate height on a substrate and then form a multilayer (ML) overthe step. However, the ML tends to smooth out the step height, leadingto a large transition area between phase-shifting and non-phase-shiftingregions. This will limit the achievable resolution limit. So it isdesired to have further improvements in this area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of a lithography system for implementing oneor more embodiments of the present disclosure.

FIG. 2 is a diagrammatic perspective view of a projection optics box(POB) employed in the lithography system for implementing one or moreembodiments of the present disclosure.

FIGS. 3-5 are diagrammatic cross-sectional views of various aspects ofone embodiment of an EUV mask at various stages of a lithography systemconstructed according to aspects of the present disclosure.

FIG. 6 is a diagrammatic perspective view of an EUV mask according toaspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper”, “over” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

Referring to FIG. 1, an EUV lithography system 10 that may benefit fromone or more embodiments of the present invention is disclosed. The EUVlithography system 10 employs an EUV radiation source 20 having awavelength of about 1-100 nm.

The EUV lithography system 10 also employs an illuminator 30. Theilluminator 30 may comprise refractive optics, such as a single lens ora lens system having multiple lenses (zone plates) and/or reflectiveoptics, such as a single mirror or a mirror system having multiplemirrors in order to direct light from the radiation source 20 onto amask 40. In the EUV wavelength range, reflective optics is employedgenerally. Refractive optics, however, can also be realized by e.g.,zoneplates. In the present embodiment, the illuminator 30 is set up toprovide a on-axis illumination (ONI) to illuminate the mask 40. In ONI,all incoming light rays incident on the mask at the same angle ofincidence (AOI), e.g., AOI=6°, as that of the chief ray. In realsituation, there may be some angular spread of the incident light. Forexample, if a disk illumination (i.e., the shape of the illumination onthe pupil plane being like a disk centered at the pupil center) of asmall partial coherence σ, e.g., σ=0.3, is employed, the maximum angulardeviation from the chief ray is sin⁻¹[m×σ×NA], where m and NA are themagnification and numerical aperture, respectively, of the imagingsystem (i.e., the projection optics box (POB) 50 to be detailed below).Partial coherence σ can also be used to describe a point source whichproduces a plane wave illuminating the mask 40. In this case, thedistance from the pupil center to the point source in the pupil plane isNA×σ and the AOI of the corresponding plane wave incident on the mask 40is sin⁻¹ [m×σ×NA]. In the present embodiment, it is sufficient to employa nearly ONI consists of point sources with σ less than 0.3.

The EUV lithography system 10 also employs the mask 40 (in the presentdisclosure, the terms of mask, photomask, and reticle are used to referto the same item). The mask 40 can be a transmissive mask or areflective mask. In the present embodiment, the mask 40 is a reflectivemask such as described in further detail below. The mask 40 includes aplurality of main features (main polygons), such as circuit patterns.The rest region without main patterns is referred to as field. The mask40 may incorporate other resolution enhancement techniques such asphase-shifting mask (PSM) and/or optical proximity correction (OPC).

The EUV lithography system 10 also employs a POB 50. The POB 50 may haverefractive optics or reflective optics. The radiation reflected from themask 40 (e.g., a patterned radiation) is collected by the PUB 50. ThePUB 50 may include a magnification of less than one (thereby reducingthe patterned image included in the radiation).

Referring to FIG. 2, an incident light ray 60, after being reflectedfrom the mask 40, is diffracted into various diffraction orders due topresence of these mask patterns, such as a 0-th diffraction order ray61, a −1-st diffraction order ray 62 and a +1-st diffraction order ray63. For lithographic imaging, purely coherent illumination is generallynot employed. Disk illumination with partial coherence σ being at most0.3 generated by the illuminator 30 is employed. In the depictedembodiment, the non-diffracted light rays 61 are mostly (e.g., more than70%) removed by, e.g., central obscuration in the pupil plane. The −1-stand +1-st diffraction order rays, 62 and 63, are collected by the PUB 50and directed to expose a target 70. Since the strength of the −1-st and+1-st diffraction order rays, 62 and 63, are well balanced, theyinterfere with each other and will generate a high contrast aerialimage. Also, the −1-st and +1-st diffraction order rays, 62 and 63, areof the same distance from the pupil center in the pupil plane, and depthof focus (DOF) is maximized.

The target 70 includes a semiconductor wafer with a photosensitive layer(e.g., photoresist or resist), which is sensitive to the EUV radiation.The target 70 may be held by a target substrate stage. The targetsubstrate stage provides control of the target substrate position suchthat the image of the mask is scanned onto the target substrate in arepetitive fashion (though other lithography methods are possible).

The following description refers to the mask 40 and a mask fabricationprocess. The mask fabrication process includes two steps: a blank maskfabrication process and a mask patterning process. During the blank maskfabrication process, a blank mask is formed by deposing suitable layers(e.g., multiple reflective layers) on a suitable substrate, The blankmask is patterned during the mask patterning process to have a design ofa layer of an integrated circuit (IC) device (or chip). The patternedmask is then used to transfer circuit patterns (e.g., the design of alayer of an IC device) onto a semiconductor wafer. The patterns can betransferred over and over onto multiple wafers through variouslithography processes. Several masks (for example, a set of 15 to 30masks) may be used to construct a complete IC device.

In general, various masks are fabricated for being used in variousprocesses. Types of EUV masks include binary intensity mask (BIM) andphase-shifting mask (PSM). An example BIM includes an almost totallyabsorptive region (also referring to as an opaque region) and areflective region. In the opaque region, an absorber is present and anincident light beam is almost fully absorbed by the absorber. Theabsorber can be made of materials containing chromium, chromium oxide,chromium nitride, aluminum-copper, titanium, titanium nitride, titaniumoxide, tantalum, tantalum oxide, tantalum nitride, and tantalum boronnitride, or any suitable matrials. In the reflective region, theabsorber is removed and the incident light is reflected by a multilayer(ML), which will be described in further detail below. A PSM includes anabsorptive region and a reflective region. A portion of the incidentlight reflects from the absorptive region with a proper phase differencewith respect to a light reflected from the reflective region to enhancethe resolution and imaging quality. The absorber of the PSM can be madeby materials such as tantalum nitride and tantalum boron nitride at aspecific thickness. The PSM can be attenuated PSM (AttPSM) oralternating PSM (AltPSM). An AttPSM usually has 2%-15% of reflectivityfrom its absorber, while an AltPSM usually has larger than 50% ofreflectivity from its absorber.

Referring to FIG. 3, an EUV mask substrate 100 comprises a materiallayer 110 made of low thermal expansion material (LTEM). The LTEM mayinclude TiO₂ doped SiO₂, and/or other low thermal expansion materialsknown in the art. The LTEM layer 110 serves to minimize image distortiondue to mask heating. In the present embodiment, the LTEM layer 110includes materials with a low defect level and a smooth surface. Inaddition, a conductive layer 105 may be deposed under (as shown in thefigure) the LTEM layer 110 for the electrostatic chucking purpose. In anembodiment, the conductive layer 105 includes chromium nitride (CrN),though other compositions are possible.

A reflective multilayer (ML) 120 is deposed over the LTEM layer 110.According to Fresnel equations, light reflection will occur when lightpropagates across the interface between two materials of differentrefractive indices. The reflected light is larger when the difference ofrefractive indices is larger. To increase the reflected light, one mayalso increase the number of interfaces by deposing a multilayer ofalternating materials and let lights reflected from different interfacesinterfere constructively by choosing appropriate thickness for eachlayer inside the multilayer. However, the absorption of the employedmaterials for the multilayer limits the highest reflectivity that can beachieved. The ML 120 includes a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair). Alternatively, the ML120 may include molybdenum-beryllium (Mo/Be) film pairs, or any materialthat is highly reflective at EUV wavelengths can be utilized for the ML120. The thickness of each layer of the ML 120 depends on the EUVwavelength and the incident angle. The thickness of the ML 120 isadjusted to achieve a maximum constructive interference of the EUV lightreflected at each interface and a minimum absorption of the EUV light bythe ML 120. The ML 120 may be selected such that it provides a highreflectivity to a selected radiation type/wavelength. A typical numberof film pairs is 20-80, however any number of film pairs is possible. Inan embodiment, the ML 120 includes forty pairs of layers of Mo/Si. EachMo/Si film pair has a thickness of about 7 nm, with a total thickness of280 nm. In this case, a reflectivity of about 70% is achieved.

A capping layer 130 is formed over the ML 120 to prevent oxidation ofthe ML. In the present embodiment, the capping layer 130 includesruthenium (Ru) with about 2.5 nm thickness.

A buffer layer 132 is formed over the capping layer 130. The bufferlayer includes ruthenium (Ru), Ru compounds such as RuB and RuSi,chromium (Cr), Cr oxide, and Cr nitride. Alternatively, the cappinglayer 130 and the buffer layer 132 can be replaced by a single layer.

In the present embodiment, an absorption stack 140 is formed over thebuffer layer 132. The absorption stack 140 preferably absorbs radiationin the EUV wavelength range projected onto a patterned EUV mask 200. Theabsorption stack 140 includes multiple film layers from a groupconsisting of tantalum (Ta), tantalum nitride (TaN), tantalum boronnitride (TaBN), titanium, aluminum oxide, aluminum-copper, palladium,molybdenum (Mo), or other suitable materials. In one embodiment, theabsorption stack 140 is formed by a first Mo layer 141 as a bottomlayer/a Ta layer 142 as a middle layer/a second Mo layer 143 as a toplayer, as shown in FIG. 3.

One or more of the layers 105, 120, 130, 132, and 140 may be formed byvarious methods, including physical vapor deposition (PVD) process suchas evaporation and DC magnetron sputtering, a plating process such aselectrode-less plating or electroplating, a chemical vapor deposition(CVD) process such as atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD), or high density plasma CVD (HDPCVD), ion beam deposition, spin-on coating, metal-organic decomposition(MOD), and/or other methods known in the art. The MOD is a depositiontechnique by using a liquid-based method in a non-vacuum environment. Byusing MOD, a metal-organic precursor, dissolved in a solvent, isspin-coated onto a substrate and the solvent is evaporated. A vacuumultraviolet (VUV) source is used to convert the metal-organic precursorsto constituent metal elements.

Referring to FIG. 4, in one of the present embodiments, the absorptionstack 140 is patterned to form the design layout pattern EUV mask 200with three states on the blank mask 100. The absorption stack 140 ispatterned to form a first state 210 having a first reflectioncoefficient r₁ by patterning technique. A patterning process may includeresist coating (e.g., spin-on coating), soft baking, mask aligning,exposure, post-exposure baking, developing the resist, rinsing, drying(e.g., hard baking), other suitable processes, and/or combinationsthereof. Alternatively, the photolithography exposing process isimplemented or replaced by other proper methods such as masklessphotolithography, electron-beam writing, direct-writing, and/or ion-beamwriting.

An etching process is performed next to remove portions of theabsorption stack 140 and form the first state 210. The etching processmay include dry (plasma) etching, wet etching, and/or other etchingmethods. In one embodiment, a multiple-step dry etching is implemented.For example, a plasma etching starts to remove the second Mo layer 143by a fluorine-based gas, then proceeds to a second etching step toremove the first Ta layer 142 by a chlorine-based gas, and then proceedsto a third etching step to remove the first Mo layer 141 by afluorine-based gas.

Referring to FIG. 5, a second state 220 having a second reflectioncoefficient r₂ on the EUV mask 200 is formed by another patterning andetching processes, similar in many respects to those discussed above inassociation with the formation of the first state 210 except it removesdifferent portion of the absorption stack 140. As an example, the secondMo layer 143 and the first Ta layer 142 are removed in the etchingprocess to form the second state 220.

Still referring to FIG. 5, now the EUV mask 200 comprises three states,210, 220 and 230. The reflection coefficients of the first state 210,the second state 220, and the third state 230 are r₁, r₂ and r₃,respectively. In the present embodiment, by choosing a properconfiguration of each layer of the absorption stack 140, such as filmmaterial and film thickness, three states can achieve prespecifiedvalues of the reflection coefficients, such as r₃=(r₁+r₂)/2. In oneembodiment, the first state 210 is configured as (in order from top tobottom) the buffer layer 132/the capping layer 130/the ML 120/the LTEMlayer 110, the second state 220 is configured as 44-nm Mo/the bufferlayer 132/the capping layer 130/the ML 120/the LTEM layer 110, and thethird state 230 is configured as 21.5-nm Mo/40-nm Ta/44-nm Mo/the bufferlayer 132/the capping layer 130/the ML 120/the LTEM layer 110. The thirdstate 230 can also be configured as 21-nm Mo/47.5-nm TaN/44-nm Mo/thebuffer layer 132/the capping layer 130/the ML 120/the LTEM layer 110 oras 21.5-nm Mo/46.9-nm TaBN/44-nm Mo/the buffer layer 132/the cappinglayer 130/the ML 120/the LTEM layer 110.

Referring to FIG. 6, the EUV mask 200 includes main polygons, such as apolygon 310 and a polygon 320, and a field 330. In the presentembodiment, state 210 has light grey background, state 220 has dark greybackground, state 230 has white background, as illustrated in the figurelegend. States 210 and 220 of the EUV mask 200 are assigned to adjacentpolygons 310 and 320, respectively. The third state 230 is assigned tothe field 330. By assigning different states to adjacent polygons andsub-resolution polygons, it will reduce the spatial frequency of maskpatterns and improve aerial image contrast and process window.

Based on the above, it can be seen that the present disclosure offersthe EUV lithography system 10. The EUV lithography system 10 employs anearly ONI, e.g., a disk illumination with partial coherence σ smallerthan 0.3 to expose an EUV mask to produce diffracted light andnon-diffracted light. The EUV lithography system 10 removes more than70% of the non-diffracted light and utilizes mainly the diffracted lightfrom two symmetrically located (on the pupil plane) and intensitybalanced −1st and +1st diffraction orders to expose a semiconductorwafer. The EUV lithography system 10 also employs an EUV mask havingthree states, namely the first state, the second state, and the thirdstate with pre-specified reflection coefficients r₁, r₂, and r₃,respectively, wherein r₃ is close to (r₁+r₂)/2. The first and the secondstates are assigned to adjacent polygons, while the third state isassigned to the field. The EUV lithography system 10 demonstrates abetter pattern contrast and throughput. The EUV lithography system 10provides a resolution enhancement technique for future nodes.

The present disclosure is directed towards lithography system andprocesses. In one embodiment, an extreme ultraviolet lithography (EUVL)system includes an extreme ultraviolet (EUV) mask with three stateshaving reflection coefficients r₁, r₂, and r₃, respectively. r₃ is closeto (r₁+r₂)/2. The system also includes a nearly on-axis illumination(ONI) with partial coherence σ less than 0.3 to expose the EUV mask toproduce diffracted light and non-diffracted light. The system alsoincludes a projection optics box (POB) to remove most of non-diffractedlight and collect and direct the diffracted light and the not removednon-diffracted light to expose a target.

In another embodiment, an EUVL process includes receiving an extremeultraviolet (EUV) mask. The EUV mask includes a first polygon, a secondpolygon adjacent to the first polygon, a field (a region withoutpolygons), a first state having reflection coefficient r₁, a secondstate having reflection coefficient r₂ and a third state havingreflection coefficient r₃. r₃ is close to (r₁+r₂)/2. The first state isassigned to the first polygon, the second state is assigned to thesecond polygon and the third state is assigned to the field. The processalso includes exposing the EUV mask by a nearly on-axis illumination(ONI) with partial coherence a less than 0.3 to produce diffracted lightand non-diffracted light, removing more than 70% of the non-diffractedlight and collecting and directing the diffracted light and the notremoved non-diffracted light by a projection optics box (POB) to exposea semiconductor wafer.

The present disclosure is also directed towards masks. In oneembodiment, an EUV mask includes a low thermal expansion material (LTEM)layer, a reflective multilayer (ML) over one surface of the LTEM layer,a conductive layer over an opposite surface of the LTEM layer, a cappinglayer over the reflective ML, a buffer layer over the capping layer andan patterned absorption stack over the buffer layer. The patternedabsorption stack defines a first, a second and a third states of theEUVL mask. Reflection coefficients of the first state, the second state,and the third state are r₁, r₂, and r₃, respectively. And r₃ is close to(r₁+r₂)/3. The mask also includes a first polygon located adjacent to asecond polygon. The first polygon has the first state and the secondpolygon has the second state. The mask also includes a field (a regionwithout polygon) having the third state.

The foregoing outlined features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An extreme ultraviolet lithography (EUVL) system,comprising: an extreme ultraviolet (EUV) mask with three states havingreflection coefficients r1, r2, and r3, respectively, wherein r3 is apre-specified value that is a function of r1 and r2; an on-axisillumination (ONI) with partial coherence σ less than 0.3 to expose theEUV mask to produce diffracted light and non-diffracted light; and aprojection optics box (POB) to remove a portion of the non-diffractedlight and to collect and direct the diffracted light and the remainingnon-diffracted light to expose a target.
 2. The system of claim 1,wherein the EUV mask comprises: a low thermal expansion material (LTEM)layer; a reflective multilayer (ML) over one surface of the LTEM layer;a conductive layer over an opposite surface of the LTEM layer; a cappinglayer over the reflective ML; a buffer layer over the capping layer; andan absorption stack over the buffer layer, wherein the absorption stackcomprises multiple layers.
 3. The system of claim 2, wherein theabsorption stack includes a top layer, a middle layer, and a bottomlayer.
 4. The system of claim 3, wherein the top and the bottom layersinclude molybdenum (Mo).
 5. The system of claim 3, wherein the middlelayer includes material from a group consisting of tantalum (Ta),tantalum nitride (TaN), and tantalum boron nitride (TaBN).
 6. The systemof claim 2, wherein the first state is configured as (from top tobottom) the buffer layer/the capping layer/the reflective ML/the LTEMlayer.
 7. The system of claim 2, wherein the second state is configuredas (from top to bottom) the bottom layer of the absorption stack/thebuffer layer/the capping layer/the reflective ML/the LTEM layer.
 8. Thesystem of claim 2, wherein the portion of the non-diffracted light is atleast 70 %.
 9. The system of claim 2, wherein the buffer layer isdisposed directly over the capping layer.
 10. The system of claim 1,wherein r3 is (r1+r2)/2.
 11. The system of claim 1, wherein the firstand the second states are assigned to adjacent polygons, while the thirdstate is assigned to a field (a region without polygons).
 12. An extremeultraviolet lithography (EUVL) process, comprising: fabricating anextreme ultraviolet (EUV) mask, including: forming an absorption stackcomprising two or more layers; removing a first portion of theabsorption stack; and removing a second portion of the absorption stack,the second portion being different than the first portion; wherein theforming, the removing the first portion, and removing the second portionresult in the EUV mask having a first state, a second state, and a thirdstate with respective pre-specified reflection coefficients r₁, r₂, andr₃, wherein r₃ is a function of r1 and r2; exposing the EUV mask by anon-axis illumination (ONI) with partial coherence σ less than 0.3 toproduce diffracted light and non-diffracted light; and collecting anddirecting the diffracted light and the not removed non-diffracted lightby a projection optics box (POB) to expose a semiconductor wafer. 13.The process of claim 12, wherein fabricating the EUV mask furthercomprises: forming a first polygon; forming a second polygon adjacent tothe first polygon; wherein the first state is assigned to the firstpolygon, the second state is assigned to the second polygon, and thethird state is assigned to a field region free of the first and secondpolygons.
 14. The process of claim 13, wherein the first polygon and thesecond polygon are circuit patterns.
 15. The process of claim 12,wherein r3 is (r1+r2)/2.
 16. The process of claim 12, wherein theabsorption stack includes a top layer, a middle layer, and a bottomlayer.
 17. An extreme ultraviolet lithography (EUVL) system, comprising:an extreme ultraviolet (EUV) mask with three states having reflectioncoefficients r1, r2, and r3, respectively, wherein r3 is approximatelyan average of r1 and r2; an on-axis illumination (ONI) with partialcoherence σ less than 0.3 to expose the EUV mask to produce diffractedlight and non-diffracted light; and a projection optics box (POB) toremove a portion of the non-diffracted light and to collect and directthe diffracted light and the remaining non-diffracted light to expose atarget.
 18. The system of claim 17, wherein the EUV mask comprises: alow thermal expansion material (LTEM) layer; a reflective multilayer(ML) over one surface of the LTEM layer; a conductive layer over anopposite surface of the LTEM layer; a capping layer over the reflectiveML; a buffer layer over the capping layer; and an absorption stack overthe buffer layer, wherein the absorption stack comprises multiplelayers.
 19. The system of claim 17, wherein the first and the secondstates are assigned to adjacent polygons, while the third state isassigned to a field (a region without polygons).
 20. The system of claim19, wherein the adjacent polygons are circuit patterns.